This present invention relates to catalyst supports and in particular, to catalysts supported on such catalyst supports, for use in processes for the hydration of olefins, e.g. in the production of ethanol or isopropanol. The present invention also relates to processes for the hydration of olefins, which employ phosphoric acid supported on such catalyst supports to catalyse the hydration reaction.
Hydration catalysts undergo ageing during operation, which is discernible by a reduction in activity and/or selectivity. Deactivation is frequently due to a reduction in the specific surface area of the support brought about by elevated temperatures. Specific surface area in the context of this application means the BET surface according to well-known method of Brunauer, Emmett and Teller determined by nitrogen adsorption according to DIN 66 132.
The specific surface area of support is closely related to its pore structure. Moreover, solids having a high surface area usually have a completely or predominantly amorphous structure, which has a strong tendency to take on a thermodynamically stable state by crystallite growth accompanied by a reduction in specific surface area.
It has been found that catalyst supports containing silicon dioxide are also subject to such ageing. Hydrothermal conditions accelerate ageing. Hydrothermal conditions prevail in chemical reactions in aqueous systems when the temperature is above the boiling point of water and pressure is above standard pressure. It is furthermore known that contaminants, in particular alkali metals, promote the ageing of supports containing silicon dioxide under hydrothermal conditions (c.f. for example R. K. Iler in The chemistry of Silica, page 544, John Wiley & Sons (1979).
EP 0 578 441 B1 describes the use of a catalyst support for the hydration of olefins. The active component, which is brought onto the support by soaking, is phosphoric acid. This particular support comprises of pellets of synthetic silicon dioxide having high crush strength, high porosity and few metallic contaminants. The purpose of the pores of the support is to accommodate the active component. Pore volume is thus preferably greater than 0.8 ml/g. Average pore radius prior to use in the hydration process is in the range between 1 and 50 nm.
In order to achieve optimum hydration performance, EP 0 578 441 B1 specifies a silicon dioxide content of the support of at least 99 wt % with below 1 wt %, preferable below 0.3 wt % of contaminants. This type of catalyst support has also been described in EP 0 393 356 B1 and in U.S. Pat. No. 5,086,031
It has surprisingly also been found that the catalyst supports based on synthetic pyrogenically produced silicon dioxide described in EP 0 393 356 B1 are also subject to ageing under hydrothermal conditions. Wherein small pores combine to yield larger pores with loss of specific surface area. Initially, pore volume remains virtually unchanged during such ageing. This ageing is unexpected because the pyrogenic silicon dioxide of which the supports consist has excellent temperature resistance according to investigations with a scanning electron microscope, the morphology of pyrogenic silicon dioxide does not change on heating to temperatures of up to 1000° C. for a period of 7 days (Schriftenreihe Pigmente Nr. 11: Grundlage von Aerosil®; Degussa publication, 5th edition, June 1993, page 20).
Klimenko (U.S. Pat. No. 3,311,568) has described the positive influence of TiO2 on the lifetime of a phosphoric acid loaded, naturally occurring siliceous support in the hydration of unsaturated hydrocarbons. At that time it was believed that natural siliceous deposits such as diatomite, kieselguhr or diatomaceous earth were the most suitable supports for these applications. However, naturally occurring siliceous materials always contain impurities that have some adverse effects on the catalytic properties. These adverse affects can be diminished, as is demonstrated in a number of patents, e.g. DE 37 09 401 A1, EP 0 018 022 B1, DE 29 29 919, DE 29 08 491, DE 1 156 772. This, however, requires a substantial number of additional steps in the support/catalyst preparation.
In order to obtain a sufficient physical strength, Klimenko had to calcine at a temperature from 1050 to 1350° C., the calcination time being between 5 and 24 hours.
Schluechter et al. (U.S. Pat. No. 5,208,195) recognise that H3PO4 containing catalysts based on synthetic silica-gels supports are highly active and possess a sufficient initial mechanical strength. However, as they state, these supports have the remaining disadvantage that the amorphous silica partially crystallises during prolonged use under conditions of the hydration reaction. This is associated with a sharp decrease in the specific surface area and hence in catalytic activity and with a decrease in mechanical strength. Because of these drawbacks, they prefer to work with naturally based siliceous materials which require a large number of preparation steps, e.g. treatment with acid in order to decrease the alumina content, until they are fit to be used as a support for hydration purposes.
Schluechter et al. describe the use of titanium dioxide in order to increase the compressive strength of catalysts spheres which are largely based on an essentially montmorillonite-containing clay, hence, a natural occurring material. The titanium dioxide is admixed with the acid treated clay and finely divided silica gel, the TiO2 content is 1.5 to 2.5 parts by weight, the content of synthetically produced silica gel is from 20 to 40 parts by weight. The mixture is optionally shaped and calcined.
It is also known from the prior art that silica which is modified by impregnation with a soluble Group IVB-compound, shows improved stability, see e.g. EP 0 792 859 A2. Titanium is one of the elements of Group IVB. The silica support is modified with the stabilising element using the impregnation process, preferably by pore volume impregnation.
Pore volume impregnation is performed by dissolving a soluble compound of the stabilising element in a volume of solvent which is equal to the pore volume of the catalyst support and then distributing the solution, for example by spraying, over the support, which may be rotated in a pill coater during spraying in order to ensure uniform impregnation.
Both aqueous and organic solvents or mixtures thereof may be used for impregnation. In industrial practice, water is generally preferred as solvent. Selection of the suitable solvent, however, is dependent upon the stabilising element compound to be used. An organic titanium compound, such as for example tetrabutoxytitanium (Ti(C4H9O)4, may also be used instead of aqueous titanium (III) chloride. In this case, butanol is a suitable solvent.
EP 0 792 859 (A2) shows that the degree of stabilisation of pyrogenic silica increases with increasing Ti-content. However, the addition of titanium leads to a decrease in pore volume, and, hence, a lower activity of the catalyst. Therefore, the need exists to keep the Ti-content as low as possible.
As is shown in the examples of the above mentioned patent application, the impregnation with aqueous solutions of TiCl3 yields materials with only limited stabilisation. At a comparable Ti-loading, the use of a Ti-alcoholate gave much better results. These are thus clearly preferred as source for Ti. Since Ti-alcoholates cannot be dissolved in water, organic solvents have to be used in order to impregnate the stabilising element. Appropriate and costly precautions must be taken to avoid any explosion hazard in the manufacturing of the support.
The modification of supports by means of impregnation with a stabilising element requires a substantial number of steps before the finished stabilised support is obtained. First of all, the support must be shaped, for instance by extrusion or by tabletting, then dried and calcined. Next, the stabilising element needs to be impregnated, then dried again. Finally, the treated supports are calcined at temperatures of between 160 and 900° C.
There is a need therefore for a less expensive and less hazardous support preparation method which, at the same time, still gives the required high degree of stabilisation and leads at the same time to a highly active and selective catalyst.